Mercury's Forms Make It Hard To Gauge Harm

In the 200,000 suburban homes where gas company officials say mercury could have leaked from old gas meters, a simple yet urgent question is forming: What effects might a couple of teaspoons of mercury have on the human body?

Preliminary test results from dozens of residents known to have been exposed to mercury when their gas meters were replaced give cause for concern, officials with Nicor and the Illinois Department of Public Health said Monday.

Although nine of ten urine tests processed so far reflect safe mercury levels, one individual had results slightly above the point where experts believe mild symptoms--such as a hand tremor--might appear. Dozens of tests are still in the works.

Levels of mercury in the household air give raise to more worries, Nicor officials said. The highest ambient levels have been about 250 times what state officials consider safe to allow evacuated residents to return to their homes. Near the floor, where mercury vapor accumulates, some readings are 10,000 times the safe level, according to Nicor officials.

Yet even such estimates of risk do not reflect real knowledge so much as guesses based on past cases of mercury exposure, experts said.

The hazard in each household ultimately may hinge on variables that are nearly impossible to measure, such as ventilation where the spill occurred, and even whether a family has pets: paws can track mercury around the house.

"Each house could be different," said Mike Moomey, a toxicologist with the state Department of Public Health. "We don't know what the initial exposures were."

So far in the affected towns from Evanston to Aurora, "no one has reported any symptoms to us," Moomey said.

Some of the uncertainty stems from the paradoxical properties of mercury's numerous forms. Some compounds containing mercury can be eaten with no ill effects. In other incarnations, a droplet is deadly.

Just one teaspoon of a very toxic form of mercury can be enough to contaminate a 25-acre lake. Yet medical experts say the familiar scenario of fish contaminated by mercury poisoning has little to do with the Nicor mercury problem.

The pure form of mercury used in the gas meters, called elemental mercury, poses less risk in general than the more lethal variety found in fish, called organic mercury or methylmercury.

Mercury expelled by coal plants, garbage incinerators or by natural processes tends to accumulate in lake and ocean sediments. In some sediments, bacteria consume the mercury and transform it into methylmercury, which then finds its way into plants and small fish.

Some large predator fish favored by humans, such as bass and pike, accumulate especially high concentrations of mercury, said Tom Hornshaw, chairman of the fish contaminant monitoring program of the Illinois Environmental Protection Agency.

"It builds up to a higher level each time you move up higher in the food chain," Hornshaw said.

An infamous case of methylmercury poisoning unfolded in Minimata Bay, Japan, in 1962. Some 400 people who ate fish tainted with mercury from an industrial plant were affected in some way, and 68 died. About 22 exposed children were born with deformities.

"Depending on its form, mercury can penetrate the placenta," said Toshio Narahashi, a professor of pharmacology at Northwestern University Medical School. "It can also appear in breast milk."

The amount of mercury that can be absorbed into the bloodstream accounts for the difference between a methylmercury disaster like the Minimata Bay incident and the suburban mercury leaks.

About 90 percent of methylmercury is absorbed into the human bloodstream, compared with just trace amounts of elemental mercury in its liquid state. Yet elemental mercury tends to evaporate over time. In its vapor state, this form of mercury becomes a major threat, with 80 percent absorbed into the bloodstream.

In 1998, two teenagers who stole about 40 pounds of elemental mercury from an abandoned neon plant in Texarkana, Ark., created one of the worst mercury contamination crises in recent years.

Nine Texarkana families had to be evacuated from contaminated homes, and six residents who came into contact with the mercury were hospitalized. In a particularly unwise move, one of the teenagers smoked cigarettes after dipping them in mercury. He was taken to the hospital after he began coughing up blood.

"In its vaporous, molecular form, the mercury can more easily cross membranes in the lungs," said Dr. Tom Kelly, a neurologist at the University of Chicago.

Some neurologists believe that Sir Isaac Newton, formulator of the theory of gravitation, suffered from mercury poisoning from experimenting with vaporous mercury. Such experiments coincided with a major personality change in his later years, from which he never recovered.